How to round out edges?
If you want to keep the flat shaded look, then you'll just have to model more edge loops and achieve the rounded look with the actual topology.

Otherwise, you might be better adding a Subsurf modifier to get smoother surfaces and curved edges. If you want to keep some edges sharper you'll need to add edge loops near the corners to keep them sharp (or you can set the Sharpness value on the polygon edges alternatively).